Dowel bars are embedded in pavement to transfer loads across a joint from one slab of concrete pavement to the next. Adjacent slabs may be created in a continuous slab of pavement by sawing a slot in the pavement before the concrete is completely set so as to result in the controlled cracking of the pavement at the desired location for a contraction joint. Once the controlled cracking occurs, the concrete acts as somewhat independent slabs lying next to one another. Adjacent slabs may also be constructed by placing a piece of compressible expansion material that extends through the full thickness of the slab at the desired location for an expansion joint. Regardless of joint type, dowel bars are typically embedded at about mid-depth of the slabs across the location of the joint. The dowel bars provide sheer strength so that a load, such as from a vehicle, is transferred from slab to slab across the joint. The dowel bars must also provide lateral movement in the longitudinal direction to allow for the slabs to move for thermal expansion and contraction. To allow the slabs to move independently, the dowel bars are coated with a bond breaker before being embedded in the concrete slab so that when the concrete hardens, the dowels will allow the slabs to slide longitudinally during thermal contraction and expansion. In order for the mechanics of the expansion and contraction to function properly, the dowels must be aligned parallel to the surface of the pavement and also parallel to the longitudinal direction of movement. The allowable tolerance for dowel bar alignment is small. Should the dowel bars not be aligned within tolerance, independent movement of the slabs may be restricted causing the slabs to lock together at the joint. Joints that are locked up create high tension stresses within the concrete pavement and can cause premature failures.
One method for installing dowel bars in pavement is to use a dowel bar assembly that includes side frame for supporting a dowel bar in the necessary orientation prior to placing a concrete slab. Typically, a dowel bar assembly is positioned and anchored in an area where two slabs of pavement will abut one another. The dowel bar assembly is then paved over with concrete, such as by slip forming or using forms and striking off the surface with a paver or screed type device. After paving, a slot is cut or formed in the surface of the pavement at the desired location which creates a weakened vertical plane to control the location of the shrinkage crack to form a contraction joint. At locations where expansion joints are desired, a compressible material such as cork or asphalt impregnated fiber board with a typical thickness of ½-inch to 1-inch, spanning the pavement width and full thickness of the pavement is made a part of the dowel bar assembly and is located at the mid-point of the dowel. This expansion material creates complete separation between adjacent slabs and provides for horizontal expansion of the pavement resulting from thermal expansion. Regardless of joint type, dowel bars are used to transfer loads between adjacent slabs of pavement and the dowel bars must be properly aligned to perform this function.
Until recently, there was no effective method for determining if dowel bars in slabs of concrete were properly aligned. Researchers at the Massachusetts Institute of Technology developed a device which uses ground penetrating radar to accurately and efficiently determine dowel bar location within a hardened slab of pavement. The use of this device is becoming more widespread, which is bringing to light the magnitude and severity of dowel bar miss-alignment. Correcting miss-aligned dowel bars requires removal and replacement of the affected pavement which is very costly and disruptive to construction projects.